The purpose of this Program Project is to conduct a series of collaborative research projects focused on its theme, "Modification and Control of Motor System Function". The Program Project contains four research proposals and four cores to address this objective. Proposals 1 and 2 utilize a newly-developed multiple single-unit recording technique that permits the simultaneous recording of eight to twelve neurons in specific central nuclei during the acquisition of various types of conditioned behavior. Proposal I focuses on the neural substrates of the classical conditioned nictitating membrane response in the rabbit. The experiments in Proposal 2 examine the role of cerebellar systems in the acquisition of two types of conditioned behavior: a conditioned modification of locomotion and an operantly-conditioned reaching behavior in chronic cats. Proposal 3 also focuses on cerebellar and brainstem systems, specifically addressing the behavioral modification of cat inferior olivary responses during the execution of specific motor behaviors. The functional characteristics of spinal reflex systems is addressed in Proposal 4. These experiments examine the segmental and supraspinal control of the foot and ankle. These projects bring together a highly interactive faculty who will share their expertise to address these research objectives in a very comprehensive manner. It is expected that the results from the proposed studies not only will provide substantial insight into the way specific components of the motor system contribute to task acquisition and motor performance, but also will reveal intriguing insights into the changes in neuronal population responses during motor learning, the characteristics of gating mechanisms occurring during motor behavior, and the interaction between descending systems and segmental spinal reflexes related to the hindlimb. The Program Project is organized in a manner that optimizes the growth of the interactions among the participating faculty and the quality of their research programs. It will be administered in an environment that is highly dedicated to training in the clinical and basic sciences and will serve as a focus for formal and informal interfaces with other divisions in the Institute and the regional academic and scientific community.